Projection lens systems (also referred to herein as "projection systems") are used to form an image of an object on a viewing screen. The basic structure of such a system is shown in FIG. 4, wherein 10 is a light source (e.g., a tungsten-halogen lamp), 12 is illumination optics which forms an image of the light source (hereinafter referred to as the "output" of the illumination system), 14 is the object which is to be projected (e.g., a LCD matrix of on and off pixels), and 13 is a projection lens, composed of multiple lens elements, which forms an enlarged image of object 14 on viewing screen 16. The system can also include a field lens, e.g., a Fresnel lens, in the vicinity of the pixelized panel to appropriately locate the exit pupil of the illumination system.
Projection lens systems in which the object is a pixelized panel are used in a variety of applications, including data display systems. Such projection lens systems preferably employ a single projection lens which forms an image of, for example, a single panel having red, green, and blue pixels.
Pixelized panels, specifically, LCD panels, come in various sizes depending upon the type of projection system in which they are to be used. Large LCD panels, e.g., panels having a diagonal of about 10.6 inches (about 270 millimeters), can be effectively employed in producing high resolution color images since such panels can have a high pixel count while still maintaining a pixel size which is large enough for reliable manufacture. In this regard, it should be noted that for a full color image from a single LCD panel, the number of pixels needed is three times that required for a monochrome image, thus making for small pixel sizes unless large LCD panels are used.
There exists a need in the art for a projection lens for use with a large pixelized panel which simultaneously has at least the following properties: (1) a long focal length; (2) the ability to operate at various magnifications while maintaining an efficient coupling to the output of the illumination system and a high level of aberration correction; (3) a relatively small size, including a relatively small number of lens elements, a relatively small barrel length, and a relatively small maximum lens diameter; (4) a high level of color correction; (5) low distortion; and (6) low sensitivity to temperature changes.
For a large pixelized panel, the use of a long focal length allows the field of view of the projection lens to be maintained in a range which facilitates aberration correction, e.g., the semi field of view of the lens can be around 25.degree..
A projection lens which can efficiently operate at various magnifications is desirable since it allows the projection system to be used with screens of different sizes and halls of different dimensions without the need to change any of the components of the system. Only the object and image conjugates need to be changed which can be readily accomplished by moving the lens relative to the pixelized panel. The challenge, of course, is to provide efficient coupling to the output of the illumination system and a high level of aberration correction throughout the operative range of magnifications.
A relatively small projection lens is desirable from a cost, weight, and size point of view. Large numbers of lens elements and elements having large diameters consume more raw materials, weigh more, and are more expensive to build and mount. Long barrel lengths normally increase the overall size of the projection system, which again leads to increased cost and weight. Accordingly, a lens with a minimum number of relatively small lens elements, located relatively close to one another, is desired.
A high level of color correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. These problems are typically most severe at the edges of the field. In general terms, the color correction, as measured at the pixelized panel, should be better than about a pixel and, preferably, better than about a half a pixel to avoid these problems.
All of the chromatic aberrations of the system need to be addressed, with lateral color, chromatic variation of coma, and chromatic aberration of astigmatism typically being most challenging. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing cathode ray tubes (CRTs) a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction is thus needed from the projection lens.
It should be noted that color aberrations become more difficult to correct as the focal length of the projection lens increases. Thus, the first and fourth criteria discussed above, i.e., a long focal length and a high level of color correction, work against one another in arriving at a suitable lens design.
The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center. Moreover, projection lenses are often used with offset panels, the lenses of FIGS. 1-3 being designed for such use. In such a case, the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but can increase monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.
In order to produce an image of sufficient brightness, a substantial amount of light must pass through the projection lens. As a result, a significant temperature difference normally exists between room temperature and the lens' operating temperature. In addition, the lens needs to be able to operate under a variety of environmental conditions. For example, projection lens systems are often mounted to the ceiling of a room, which may comprise the roof of a building where the ambient temperature can be substantially above 40.degree. C. To address these effects, a projection lens whose optical properties are relatively insensitivity to temperature changes is needed.
One way to address the temperature sensitivity problem is to use lens elements composed of glass. Compared to plastic, the radii of curvature and the index of refraction of a glass element generally change less than those of a plastic element. However, glass elements are generally more expensive than plastic elements, especially if aspherical surfaces are needed for aberration control. As described below, plastic elements can be used and temperature insensitivity still achieved provided the powers and locations of the plastic elements are properly chosen.
The projection lenses described below achieve all of the above requirements and can be successfully used in producing relatively low cost projection lens systems capable of forming a high quality color image of a pixelized panel on a viewing screen.